1. Field of the invention
The present invention relates to an antilock brake control apparatus, and more specifically to an antilock brake control apparatus that adjusts the wheel cylinder pressure by means of an actuator comprising an on/off valve. By applying open-loop control of the wheel cylinder pressure, the pulsating effects of the on/off-type valve can be reduced and the wheel cylinder pressure can thus be controlled with high precision.
2. Description of the prior art
In an antilock brake control apparatus according to the prior art, wheel skidding is detected from the acceleration and deceleration of the wheels and the relationship between the detected wheel speed and the estimated vehicle speed. The wheel cylinder pressure is then adjusted according to the skidding of the detected wheel to maintain wheel skidding to an appropriate level, i.e., within a limited range of the peak friction coefficient of the road surface. This control process makes it possible to shorten the total braking distance and maintain vehicle and steering stability.
Antilock brake control systems of this type generally use a solenoid valve or other valve as an on/off valve to regulate brake pressure. As a result, the de/pressurization characteristics of the wheel cylinder pressure relative to the operating (opening and closing) time of the on/off valve vary according to an exponential function because the increase/decrease characteristics of the wheel cylinder pressure depend upon the current wheel cylinder pressure and the pressure differential between the master cylinder pressure and the wheel cylinder pressure. It is therefore necessary to determine as precisely as possible the change in the wheel cylinder pressure in order to control the wheel cylinder pressure with high precision.
However, because of the high pressure pulses accompanying operation of the on/off valve, it is extremely difficult to control the wheel cylinder pressure with high precision insofar as the on/off valve control signal is output within a closed-loop process based on sequential comparison of the actual wheel cylinder pressure and the target wheel cylinder pressure. In other words, the wheel cylinder pressure may fluctuate along a high amplitude wave due to the pulses resulting from valve operation. As a result, the wheel cylinder pressure measured by the pressure detection means mounted on the wheel cylinder at any given point during antilock brake control may not accurately express the precise actual wheel cylinder pressure at that time. It is therefore difficult to precisely control wheel cylinder pressure and braking by sequential comparison of the target and supposed "actual" wheel cylinder pressures detected by such pressure detection means.